1. Field of the Invention
The invention relates to color measuring instruments and more particularly to spectrophotometers of a type that employ an integrating sphere in measuring light reflected from an object sample under test.
2. Prior Art
Spectrophotometers are instruments used to determine the color of an object under test and typically include a source of light projected onto the object, a photodetector detecting light reflected from the object sample and signal processing circuitry connected to the photodetector to compute curves or numerical values indicative of color. The general principles of construction and use of these instruments are well known and understood by those skilled in the art and are described in published texts such as the text by Richard S. Hunter entitled The Measurement of Appearance, John Wiley & Sons, 1975.
One known type of spectrophotometer uses an integrating sphere in which the light reflected from the object is integrated to obtain an average reading of the color over an exposed surface area of the object. Known integrating spheres can provide readings which represent "total" reflections or "diffuse-only" reflections. The total reflections include all reflections from the exposed object, including reflections from the surface and diffuse reflections from particles in the body of the object. The "diffuse-only" measurement is obtained by excluding the first surface reflections which are reflected at the specular angle and may be referred to as the specular component. Exclusion of the specular component eliminates the light contribution due to gloss, and the color values obtained from a specular-excluded reading are independent of the glossiness of the surface of the object under test.
The specular angle is equal and opposite to the angle of incidence of a source light beam projected onto the object under test. A typical angle for the source light beam in prior art arrangements is 8 degrees off the vertical centerline of the sphere. The specular-excluded reading is obtained in such a sphere by providing a light-absorbing area on the sphere at 8 degrees off the centerline and on the opposite side of the centerline from the light source. The absorbing spot may be created by a opening in the outer surface of the sphere with a dark area or black plug placed in the opening to absorb the specular component.
Prior art integrating spheres used in the photospectrometers are commonly quite large, for example, 6 to 8 inches in diameter. The size of the sphere is dictated to some extent by the diameter of the measuring opening adjacent to the object under test and the space in the sphere surface for a light source, the specular absorption area, and detectors, while allowing sufficient interior surface area for integrating action in the sphere. Because the prior art spheres are relatively large, and additional space is needed for lenses associated with the light source and detection and analysis or signal processing apparatus, the typical prior art photospectrometer is a bench unit which is awkward to move and objects to be tested must be brought over to the bench unit. The need for an accurate portable spectrophotometer is readily apparent. Portable spectrophotometer may, for example, be used to measure the color of paint on the wall of a large auditorium, on automobiles, bolts of textile, etc., without the need to move portions or sections of these objects to a bench photospectrometer. The problem in prior art systems is that no practical, small-diameter, integrating sphere has been available to measure both specular-included and specular-excluded sample effects on a relatively large sample area.